The geometries and energies of the electronic states of
phenyloxenium ion(Ph-O+)
were computed at the multireference CASPT2/pVTZ level of theory. Despite being isoelectronic to
phenylnitrene , the phenyloxenium ion has remarkably different energetic orderings of its
electronic states. The closed-shell singlet configuration (1A1)
is the ground state of the phenyloxenium ion , with a computed adiabatic energy
gap of 22.1 kcal/mol to the lowest energy triplet state (3A2). Open-shell singlet configurations (1A2,
1B1, 1B2, 21A1)
are significantly higher in energy (> 30 kcal/mol) than the closed-shell
singlet configuration. These values suggest a revision to the current
assignments of the ultraviolet photoelectron spectroscopy bands for the phenoxy
radical to generate the phenyloxenium ion . For para-substituted phenyloxenium ions, the adiabatic
singlet-triplet energy gap (DEST)
is found to have a positive linear free energy relationship (LFER) with the
Hammett-like s+R/s+ substituent parameters; for
meta substituents, the relationship is non-linear and negatively
correlated. CASPT2 analyses of the
excited states of p-amino
phenyloxenium ion and p-cyano phenyloxenium ion indicate that the relative orderings of
the electronic states remain largely unperturbed for these para substitutions. In
contrast, meta-donor substituted phenyloxenium ions have low-energy open-shell
states (open-shell singlet, triplet) due to stabilization of a p,p* diradical state by the donor substituent. However, all of the other phenyloxenium
ions and larger aryloxenium ions (naphthyl, anthryl) included in this study have closed-shell singlet ground
states. Consequently, ground-state reactions of phenyloxenium ions are
anticipated to be more closely related to closed-shell singlet arylnitrenium
ions (Ar-NH+) than their isoelectronic arylnitrene (Ar-N)
counterparts.

2.Theoretical
Investigation of Heteroaryl Oxenium Ions

The
electronic state orderings and energies of heteroaryl oxenium ions were
computed using high-level CASPT2//CASSCF computations. We find that these ions have a number
of diverse, low-energy configurations.
Depending on the nature of the heteroaryl substituent, the lowest-energy
configuration may be open-shell singlet, closed-shell singlet, or triplet, with
further diversity found among the subtypes of these configurations. The 2- and 3-pyridinyl oxenium ions
show small perturbations from the phenyl oxenium ion in electronic state orderings
and energies, having closed-shell singlet ground states with significant gaps
to an n,p* triplet state. In contrast, the 4-pyridinyl oxenium
ion is computed to have a low-energy nitrenium ion-like triplet state. The pyrimidinyl oxenium ion is computed
to have a near degeneracy between an open-shell singlet and triplet state, and
the pyrizidinyl oxenium ion is computed to have a near triple degeneracy
between a closed-shell singlet state, an open-shell singlet state, and a
triplet state. Thus, the ground
state of these latter heteroaryl oxenium ions cannot be predicted with
certainty; in principle, reactivity from any of these states may be
possible. These systems are of
fundamental interest for probing the spin and configuration-dependent
reactivity of unusual electronic states for this important class of reactive
intermediate.

3.Experimental
Generation and Reactivity of the Phenyloxenium Ion

Photolysis of protonated
phenylhydroxylamine was studied using product analysis, trapping experiments,
and laser flash photolysis experiments (UV-Vis and TR3 detection) ranging from
the femtosecond to the microsecond timescale. We find that the singlet excited state of the photoprecursor
partitions within 5 ps into two channels, generating both a longer-lived
low-wavelength absorbing transient (5 ns) that we assign to the phenoxy radical
and a very short-lived (80 ps) longer-wavelength absorbing transient that we
assign to the phenyloxenium ion.
Product studies from photolysis of this precursor show rearranged
protonated o/p-aminophenols and solvent water adducts (catechol,
hydroquinone). The former products
can be largely ascribed to radical recombination or ion recombination, while
the latter are ascribed to solvent water addition to the phenyloxenium
ion. The phenyloxenium ion is
apparently too short-lived under these conditions to be trapped by external
nucleophiles other than solvent, giving only trace amounts of chloro and azido
adducts upon addition of chloride and azide traps, respectively. Product
studies upon thermolysis of this precursor give the same products as those
generated from photolysis, with a difference being that the ortho adducts
(o-aminophenol, o-hydroquinone) are the major products from heating (higher
ratio), whereas the para adducts are the major products from photolysis.